The present invention relates to multichannel signal measuring. More particularly, the present invention relates to a novel and improved method for collecting multichannel signals comprising of the signal of interest and of superposed background interference contributions which may be much larger than the signal of interest.
Performing many simultaneous measurements on a subject, i.e. multichannel detection, is sometimes essential in order to obtain sufficient information on the issue under examination. We consider, in particular, the detection of biomagnetic fields associated with the function of human brain or heart. Modern magnetometers for this purpose comprise about 100 channels to enable accurate localization of neuro- or cardiographic sources. Biomagnetic fields are very weak in comparison to the background magnetic fields in the surroundings, so that the problem of resolving the real signal from environmental interference is technically very challenging (M. Hxc3xa4mxc3xa4lxc3xa4inen et. al., xe2x80x9cMagnetoencephalographyxe2x80x94theory, instrumentation, and applications to noninvasive studies of the working human brainxe2x80x9d, Rev. Mod. Phys. vol. 65, no 2 April 1993.).
Prior art of protecting very sensitive instruments against external interference include basically five methods: 1) use of passive shielding elements surrounding the instrument (magnetically shielded room in the biomagnetic application), 2) use of active elements canceling the interfering environmental signal (large scale compensation coils in magnetic measurements), 3) reducing the relative sensitivity of the sensors to typical background signals (use of gradiometers instead of magnetometers), 4) use of additional sensors to estimate the background interference in order to separate it out from the signals, and 5) numerical processing of the multichannel data to separate true signal from external interference.
In method 1), when applied to biomagnetic measurements, the instrument is placed inside a shielding room having walls made of high permeability metal alloy (mu-metal). In the low frequency range, relevant to biomagnetic signals, the shielding factor of such a room is limited to about 100-1000 by reasonable amount and finite permeability of mu-metal. At high frequencies the shielding may be improved by adding layers of highly conducting material, such as aluminium (V. O. Kelh{overscore (a)} et. al., xe2x80x9cDesign, Construction, and Performance of a Large-Volume Magnetic Shieldxe2x80x9d, IEEE Trans. on Magnetics, vol. MAG-18, no 1, January 1982.).
In studies of human subjects, possibly patients in a hospital, the magnetically shielded room has to be relatively large, leading to a heavy and expensive construction. Sufficient shielding requires multilayer structure with total wall thickness of about 0.6 m. Thus, the outer dimensions of the room must be on the order of 4 mxc3x975 mmxc3x973.5 m to provide enough space for the instrument and comfortable conditions for the patient on a bed, and possibly for medical personnel taking care of the patient. Especially, the need of 3.5 m in height (two floors) is inconvenient in a typical hospital environment.
Method 2), when large compensation coils are used (EP 0 514 027, M. Kazutake et al. xe2x80x9cMagnetic noise reducing device for a squid magnetometerxe2x80x9d) resembles the passive shielding with high permeability material. The shielding current, which in mu-metal is generated as a natural response to an exposure to magnetic field, is now generated artificially in a control system and driven into coils with dimensions comparable to those of a typical shielded room. As a realization of such a system, three orthogonal Helmholtz pairs may be used. The external field to be eliminated is measured outside the coil system by field sensing elements, such as fluxgates, whose output is converted by a proper control system into electrical currents fed into the compensation coils. This kind of active shielding is far lighter and less expensive than a typical passive shield. It also performs best at low frequencies, where passive shielding of magnetic fields is most difficult.
The major disadvantage of method 2) is the very restricted geometry of the shielding currents. In practice, a compensation coil system can reject the field of distant sources only, which produce nearly uniform field at the site of the instrument. It may also be difficult to find the optimal positions for the field sensing elements, and if the environmental conditions change, the system may have to be readjusted.
Method 3), regarding the biomagnetic application, is based on the fact that the gradients of a magnetic field decrease more rapidly as a function of the distance from the source than the field itself. Therefore, the signal to background ratio is increased by measuring the difference of magnetic flux between two adjacent locations instead of the flux itself: the signal arising from the nearby object of study (e.g. a brain) is enhanced in comparison to the disturbance signal from an interfering source further away.
In principle, method 3) provides total immunity against uniform interference fields. In practice, however, the balance of best gradiometers is limited to at best 1/1000 because of technical difficulties in controlling the geometry of the sensors. In addition, the interfering fields are never strictly uniform. If the disturbing source is located a distance l away (typically 1-10 m) and the baselength of the gradiometer is h (typically 0.01-0.1 m), the background signal of the sensor is damped roughly by a factor of h/l compared to a magnetometer with the same loop size.
The most severe drawback of method 3) is that it rejects part of the signal arising from the object of study as well. This is especially unfavorable when the biomagnetic field is nearly uniform on the length scale of the sensor. This is to some extent the case in cardiac studies, and when a neuromagnetic source is located deep below the scull. For this reason, magnetometers would be preferred instead of gradiometers in many biomagnetic measurements (M. Hxc3xa4mxc3xa4lxc3xa4inen et. al., xe2x80x9cMagnetoencephalographyxe2x80x94theory, instrumentation, and applications to noninvasive studies of the working human brainxe2x80x9d, Rev. Mod. Phys. vol. 65, no 2 April 1993).
In method 4) (U.S. Pat. No. 5,187,436 A, J. A. Mallick xe2x80x9cNoise cancellation method in a biomagnetic measurement system using an extrapolated reference measurementxe2x80x9d, and U.S. Pat. No. 5,020,538, N. H. Morgan et al., xe2x80x9cLow Noise Magnetoencephalogram system and methodxe2x80x9d, and DE 4131947, G. M. Daalmans, xe2x80x9cMehrkanalige SQUIDxe2x80x94Detektionseinrichtung mit St{overscore (o)}rfeldunterdr{overscore (u)}ckungxe2x80x9d, and DE 4304516, K. Abraham-Fuchs, xe2x80x9cVerfahren zum Bestimmen einer Characteristischen Feldverteilung einer ortsfesten St{overscore (o)}rquellexe2x80x9d, and WO 93/17616, K. Abraham-Fuchs, xe2x80x9cDisturbances suppression process during position and/or direction finding of an electrophysiological activityxe2x80x9d, and EP 0481 211, R. H. Koch, xe2x80x9cGradiometer having a magnetometer which cancels background magnetic field from other magnetometersxe2x80x9d, and U.S. Pat. No. 5,657,756, J. Vrba et al., xe2x80x9cMethod and systems for obtaining higher order gradiometer measurements with lower order gradiometersxe2x80x9d) the apparatus is equipped with additional background sensors, which are so arranged that they do not receive any substantial input from the object of study. They are usually placed further away from the actual sensor array. From the signals of these sensors an estimate of the interfering background field is calculatedxe2x80x94for example up to the desired order in the Taylor expansion of the fieldxe2x80x94and then properly extrapolated and subtracted from the signals of the actual measuring channels.
The relatively large distance between the background sensors and the actual sensors and the inaccuracy in the calibration and relative location and orientation of the sensors are the main drawback of this method, because these factors limit the degree of achievable compensation. Especially, correct interpretation and use of the background sensor outputs is practically impossible, for example, if the background signal arises from an unknown vibration mode of the instrument in an unknown remanence field distribution.
In method 5) the signals collected by a multichannel device during a measurement are first stored on a memory device. After the measurement, the data are processed with a numerical template or projection method to separate out the contributions of the interesting biomagnetic sources from the disturbing interference fields (WO 94/12100, R. Ilmoniemi, xe2x80x9cMethod and apparatus for separating the different components of evoked response and spontaneous activity brain signals as well as of signals measured from the heartxe2x80x9d, and WO 93/17616, K. Abraham-Fuchs, xe2x80x9cDisturbances suppression process during position and/or direction finding of an electrophysiological activityxe2x80x9d, and U.S. Pat. No. 4,977,896, S. E. Robinson et al., xe2x80x9cAnalysis of biological signals using data from arrays of sensorsxe2x80x9d).
Method 5) relies on the multichannel aspect of the measuring device: only by collecting data from many sensors simultaneously can the background interference be separated from the true signal due to their characteristically different distribution over the entirety of channels. In a neuromagnetic measurement, for example, the sensors should cover the whole head.
When used with magnetometers method 5) requires very large dynamic range for every channel of the data collection system, since the actual biomagnetic signal can be contaminated by a background signal several orders of magnitude larger. One would effectively have to subtract large but nearly equal numbers from each other to reveal the differences representing the actual biomagnetic activity.
In practice, the strength of biomagnetic fields is 6-8 orders of magnitude weaker than the unshielded background fields in a typical environment. (M. Hxc3xa4mxc3xa4lxc3xa4inen et. al., xe2x80x9cMagnetoencephalographyxe2x80x94theory, instrumentation, and applications to noninvasive studies of the working human brainxe2x80x9d, Rev. Mod. Phys. vol. 65, no 2 April 1993). Therefore, at least two of the above methods have to be combined to achieve a tolerable signal to background ratio.
In the present invention a multichannel sensor device is made immune to environmental interference by cross coupling the channels in such a way that there is no output in response to the interference. No extra compensation or reference channels are necessary. In practice it turns out that the shielding efficiency of the present method is proportional to the number of cross coupled channels and therefore the present method where all or most of the signal channels participate the compensation is superior to prior art methods which utilize a smaller number of separate compensation channels (e.g. U.S. Pat. No. 5,657,756, J. Vrba et al., xe2x80x9cMethod and systems for obtaining higher order gradiometer measurements with lower order gradiometersxe2x80x9d).
Ordinarily, when N channels are operated in parallel, the output of each channel depends on the input of its own sensor only. This can be described by a diagonal Nxc3x97N matrix C, by which the N-dimensional output vector U is obtained as
U=Cu
for a given N-dimensional input vector u. The element Cii of the coupling matrix is the gain, or the calibration constant, of the respective channel i.
The present compensation method is described by a non-diagonal matrix C, whose off-diagonal components represent the cross couplings between the channels. This matrix is constructed so that it maps to null vector all the input vectors interpreted as interference; the required linear mapping C in N-dimensional signal space has n-dimensional null-space, where n is the number of independent interference vectors, or field distributions, spanning the subspace called interference signal space. In practical applications n less than  less than N.
The prior art invention WO 94/12100 (R. Ilmoniemi, xe2x80x9cMethod and apparatus for separating the different components of evoked response and spontaneous activity brain signals as well as of signals measured from the heartxe2x80x9d) describes a signal space method where different biomagnetic responses are separated from each other and from interference originating from uninteresting sources by applying signal space projection methods to collected data. Typical interference signalsxe2x80x94especially in the case of magnetometersxe2x80x94may be by factor 10000 or 1000000 larger than the signals of interest. Therefore the aforementioned prior art method would require data collection and storage with too much extra dynamic range to be practical.
The required cross-coupling strengths for the sensor network in the present method are determined from a measurement of the interference seen in the absence of the cross couplings. For the determination of the cross coupling strengths no detailed information on the location, orientation, or calibration of individual channels or their relative locations and orientations is required. Full compensation of an N-channel system can be accomplished by 2Nn cross couplings, when the couplings are realized by negative feedback. In practice even a lower number of couplings may be sufficient.
When performing a measurement with the compensated system the magnitudes of the background interference signal components in the interference signal space are recorded together with the compensated signals and, if required, the uncompensated signals can be recovered from this information with a linear transformation.
The use of the present method in the biomagnetic application is in a way analogous to using a shielded room: The sensors of the multichannel magnetometer detect the interference due to the field in the magnetically shielded room and n xe2x80x9cshielding currentsxe2x80x9d are constructed from this information and then delivered, properly weighted, to the individual channels in form of negative feedback. This negative feedback is superimposed on the ordinary negative feedback used to drive the magnetometer channels in the flux locked loop. The same feedback coils used for the flux locking negative feedback can be used to feed in the shielding currents as well.
As to prior art methods 1) and 3) the invention effectively improves magnetic shielding and thus enables to use sensitive magnetometer sensors instead of gradiometers in a standard shielded room. The present method for eliminating the interference is adaptive to the conditions present at a particular site, since the cross couplings are chosen to cancel the interference measured by the very sensor array itself. The effective shielding factor so achieved is comparable to that of the best balanced gradiometers of the date.
As to prior art method 2) the present invention offers more flexible adaptive shielding. The N negative feedback coils of the individual channels replace the small number of large, fixed geometry external compensation coils of method 2). In our method the compensation currents have no interaction with the walls of the shielding room. Also, any vibration of the magnetometer array in the remanence field is impossible to handle with a set of external compensation coils but in the present method it is simply an extra dimension in the interference signal space.
As to prior art method 4) the present invention offers several advantages: No extra compensation or reference sensors are necessary. For successful compensation there is no need to accurately calibrate or balance sensors or to make any sensors parallel or orthogonal to each other. Neither is the compensation limited to any order in the Taylor expansion of the interfering magnetic field. In the present method the interference is simply compensated up to any order necessary; the degree of compensation achieved depends only on how accurately the devices used for setting the cross coupling strengths can be set. The degree of achievable compensation is also proportional to the number of channels participating in the cross coupled network. In the present method this number can be freely chosen and can be increased up to the total number of channels in the device (N) instead of the relatively limited number of separate compensation channels used in the prior art compensation schemes. A standard compensation obtained by adding or subtracting a reference signal increases the noise in the signals. Such increase of noise is absent from the present method because the linear mapping C is a projection.
A reduction in the number of actual measurement channels takes place in method 4) when out of the N sensors of the system n are chosen permanently to be compensation sensors and moved further away from the source of the actual signals. The present method is more flexible at this point because the number of compensated interference modes n can be chosen according to the needs dictated by the environment and the quality of the shielded room.
Also, in method 4) the quality of compensation may suffer if any one of the n compensators does not work properly. In the present this problem is absent because the individual channels are equal and a malfunctioning channel can simply be disconnected from the feedback network (and discarded from the data).
As to prior art method 5) the advantage is that no sensors or data collection devices with excess dynamic range are needed. The xe2x80x9cshielding currentsxe2x80x9d created in the cross coupled sensor network are distributed to balance all the N sensors against the interference.
The object of the present invention is to eliminate the problems and the disadvantages described above.
A specific object of the present invention is to disclose a completely new type of method and device for eliminating background interference from multichannel signal measurement.
The approach of the present invention can be used in conjunction with measurements by any multichannel device, susceptible to environmental interference. The preferred embodiment is directed toward an application for biomagnetic measurements: magnetoencephalography (MEG) and magnetocardiography (MCG). The method, however, is more generally applicable, as long as the environmental background signal has sufficiently different characteristics from the signal of interest. This condition can usually be fulfilled by a properly arranged sensor array with a sufficiently large number of channels. For example, in neuromagnetic measurements this is accomplished by a whole head coverage with about 100 sensors.